The Silurian Period – Fish Jaws and Upright Plants

440 to 415 Million Years Ago

 

The melting of the Ordovician glacial ice produced a warm, stable climate during the Silurian, providing the perfect conditions for Life to flourish. Silurian plants developed a vascular structure giving their stalks the strength to grow as high as three feet without the support of a watery environment. This new vascular structure also allowed them to transport nutrients to both stem and stalk. And while they were as yet leafless, these early plants were able to photosynthesize their nutrients through their stems.

Plants in the Early Silurian Period thrived under ever-increasing levels of carbon dioxide (plants require CO2 for photosynthesis). As plants spread upon the landscape they produced oxygen as a byproduct of their metabolic processes. Throughout the Silurian the oxygen levels continued to increase, causing problems for plants which suffer when oxygen levels are high. By the Middle Silurian, the CO2 levels began to fall and the plants began to endure stresses which forced them to either to adapt or perish.

But while the plants were straining to cope with high oxygen, the animal kingdom was itself expanding. Hiding among the new plants we find the first absolute evidence of animal life on land consisting of fossilized insects such as millipedes and spiders.

The appearance of the first freshwater fish is notable because it tells us that there was indeed fresh water inhabited by a variety of aquatic animals; furthermore, fish, which up to this time had no jaws, now appear with jaws. Although this new biotechnology remained relatively insignificant throughout the Silurian, those species with jaws for biting and chewing would soon dominate the seas.

Oxygen, Carbon Dioxide and the Atmosphere

• Oxygen levels started out low and continually increased throughout the period, reaching levels significantly higher than today.
  
• Carbon Dioxide levels also increased for a short time but then plummeted by the end of the period.
  
• While high CO2 is beneficial for plants, high oxygen levels can cause stress and even death to plants.
  
• High oxygen levels are thought to be the leading factor enabling vertebrates to populate the landscape; and it is thought that the rapid diversification of plants contributed considerably to the supply of oxygen.
  

Geography – Continents and Oceans

• Shallow seas still covered much of the land.
  
• Most of North America lay submerged under the shallow waters of the Panthalassic Ocean.
  
• The large supercontinent Gondwana continued its slow drift southwards. Meanwhile other continents drifted towards each other, soon to form another supercontinent called Laurasia near the equator.
  
• The collision of continents led to the buildup of mountain chains that, by the end of the period, had nearly eroded away.
  
• Coral reefs became a significant presence in the Silurian seas providing an abundance of habitat for marine animals and plants alike.
  

Climate

• The Silurian climate was warm; while at the same time glaciers resided at the poles.
  
• Arid areas, many of which were in the middle of continents far from the shallow seas, tell us that the climatic mechanisms were providing a wide range of habitats.
  

Plant Life

• Some scientists feel that the plant development in the Silurian is equivalent to the Cambrian Explosion for animal life.
  
• Vascular plants appeared – these are plants whose internal structures allow them to hold themselves upright as well as transfer water and nutrients to all their different parts.  (Aquatic plants use the buoyancy of water to keep themselves upright.)
  
  • Cooksonia is a good example of these developments
• Plants played a significant role in increasing the oxygen present in the atmosphere.
  
• Plants both benefited and suffered from the changing levels of CO2 and oxygen.
  
• Plants provided habitat for the first land animals including millipedes, scorpions and other primitive insects.
  

Animal Life

• First jawed fish occurred.
• First freshwater fish appeared.
• Though appearing in the Ordovician, coral reef systems were now widespread and appeared in vast reefs, providing new habitat for a variety of creatures and prompting new species to develop.
  
• First clear evidence of land animals includes millipedes, spiders and centipedes. It is thought that they found good habitat hiding amongst the new vegetation.
  
• The top predator of the Silurian seas was the “sea scorpion” or eurypterid which reached lengths up to 9 feet long and eventually inhabited shallow waters and swamps.
  
• Trilobites were scarce during the Silurian